title: Phase shift spectra of a fiber-microsphere system at the single photon level.
abstract: We succeeded in measuring phase shift spectra of a microsphere cavity coupled with a tapered fiber using a weak coherent probe light at the single photon level. We utilized a tapered fiber with almost no depolarization and constructed a very stable phase shift measurement scheme based on polarization analysis using photon counting. Using a very weak probe light (n = 0.41), we succeeded in observing the transition in the phase shift spectrum between undercoupling and overcoupling (at gap distances of 500 and 100 nm, respectively). We also used quantum state tomography to obtain a 'purity spectrum'. Even in the overcoupling regime, the average purity was 0.982 ± 0.024 (minimum purity: 0.892), suggesting that the coherence of the fiber-microsphere system was well preserved. Based on these results, we believe this system is applicable to quantum phase gates using single light emitters such as diamond nitrogen vacancy centers.
authors.affiliation: Research Institute for Electronic Science, Hokkaido University, Sapporo 001–0020, Japan.
authors.forename: Akira
authors.initials: A
authors.lastname: Tanaka
authors.forename: Takeshi
authors.initials: T
authors.lastname: Asai
authors.forename: Kiyota
authors.initials: K
authors.lastname: Toubaru
authors.forename: Hideaki
authors.initials: H
authors.lastname: Takashima
authors.forename: Masazumi
authors.initials: M
authors.lastname: Fujiwara
authors.forename: Ryo
authors.initials: R
authors.lastname: Okamoto
authors.forename: Shigeki
authors.initials: S
authors.lastname: Takeuchi
mesh_terms: D005336:Fiber Optic Technology; D008863:Microspheres; D010783:Photometry / Q000379:methods*; D017785:Photons; D013057:Spectrum Analysis / Q000379:methods*